Hydrocarbon feedstock containing higher molecular weight hydrocarbons is cracked by contacting it under elevated temperatures with a cracking catalyst whereby light distillates such as gasoline are produced. However, the cracking catalyst gradually deteriorates during this process. One source for this deterioration is the deposition of contaminating metals such as nickel, vanadium and iron on the catalyst which increase the production of hydrogen and coke. At the same time the conversion of hydrocarbons into gasoline is reduced by these metals. It would, therefore, be desirable to have a modified cracking catalyst available, the modifying agent of which passivates those undesirable metal deposits on the cracking catalyst.